A type-casting bug was introduced by 5286a42ae841fdb91d61e62e1bc61d0f9c553780

When retrieving objects from a field, say:

class Bacon
field :smokiness, :type => SmokinessLevel
end

Mongoid is not converting the type back from a Float, Array, or whatever the MongoDB type is, back into a SmokinessLevel. The same problem is affecting standard ruby objects, like Date, DateTime, Time, and BigDecimal.

This commit fixes it, although I do have a few questions around our naming. Feedback is appreciated!

Duplicate reports: MONGOID-797, MONGOID-802, MONGOID-809